1. Field of the Invention
This invention relates to the colorimetric determination of ethyl alcohol in human breath samples. More specifically, the invention is a solid substance which changes color when exposed to a gas containing ethanol vapor.
2. Description of the Related Art
It is widely recognized that blood alcohol content may be directly correlated to the alcohol concentration of a person's alveolar breath. Law enforcement agencies commonly rely on this relationship to determine the blood alcohol content (BAC) of a suspect by using the well-known "Breathalyzer." This device is an apparatus for analyzing the alcohol content of human breath. In operation, a 50-mL sample of breath is collected in a heated reservoir and is then bubbled through an ampoule containing a dilute solution of potassium dichromate in 50% sulfuric acid. The reduction of the dichromate by alcohol in the breath sample is accompanied by a color change in the acid dichromate reagent. This color change is measured photometrically. Although the apparatus is sufficiently accurate for practical purposes, it is relatively complex, expensive, and the liquid reagent employed is highly corrosive and great care must be taken in handling the ampoules and in disposing of them after use. All these factors militate against its use except by well-trained operators under controlled conditions. This is unfortunate, since a need exists for a reliable, accurate, simple and inexpensive BAC device which could be used by motorists and other members of the general public.
In addition to the "Breathalyzer," a number of other devices exist for estimating BAC based on the color change produced when ethanol is oxidized by hexavalent chromium compounds under acidic conditions. Generally speaking, the color change is from yellow to blue-green or violet, reflecting a change from chromate ions to chromic ions. With respect to general chromium ion chemistry, chromic ions (CR.sup.+3) usually exist in a pH-dependent equilibrium with chromite ions (CrO.sub.2.sup.-) while dichromates (Cr.sub.2 O.sub.7.sup.=) exist in a pH-dependent equilibrium with chromates (CrO.sub.4.sup.=). Chromates are yellow; dichromates are orange. Chromic acid (H.sub.2 CRO.sub.4), the hydrate of CrO.sub.3, exists only in solution or as chromate salts. In an acid solution, a redox reaction will result in Cr.sub.2 O.sub.7.sup.= (orange) being reduced to Cr+3 (green or blue-green).
In some instances, silica gel has been used as a support for the acid chromate. This provides a solid phase reagent system which reduces the hazard associated with handling solutions in strong, concentrated acids. An example of a device employing this system is the Kitagawa tube, which is described in Wright, U.S. Pat. No. 3,238,783, as consisting of a narrow-bore glass tube containing powdered silica as a carrier for the reagent. The reagent is described as an anhydrous form of chromic acid which is normally yellow but changes to bluish-grey upon reduction by alcohol. It is further said that in such a tube, the length of the stain gives a measure of the quantity of alcohol passed through it.
Etzlinger, U.S. Pat. No. 3,303,840, describes an apparatus which uses a similar tube for estimating BAC from a breath sample. After collection, the breath sample is forced through a tester tube containing potassium dichromate. During this passage, the alcohol contained in the gas reduces the yellow dichromate to a greenish chromium-containing product. The location along the tube of the separation between the yellow part and the greenish part of the contents of the testing tube is compared against a graduated scale to read the alcohol content in the blood of the person providing the breath sample.
McConnaughey, U.S. Pat. No. 3,544,273, describes still another sampling device for obtaining alveolar breath samples. The collected sample is analyzed for alcohol by discharging it through a colorimetric detector tube which is said to be a glass tube containing a bed of reagent that changes color in response to contact with the alcohol in the sample. When the sample is passed through the tube "a color change occurs lengthwise of the bed, the length of the color-changed portion being dependent on the amount of detectable gas in the sample." An especially suitable indicator is said to be disclosed in U.S. Pat. No. 3,455,654 in which a hexavalent chromium compound and a pentavalent phosphoric acid are supported on an inert carrier. This indicator is said to change color from yellow to dark green on reaction with ethyl alcohol. A typical illustrative formulation contains 0.53 gram of chromium trioxide and 1.67 grams of metaphosphoric acid carried on 100 mL of 8-14 mesh silica gel.
As noted earlier, it is a common shortcoming of the above-described BAC devices and systems that they involve color changes which are not always easy to perceive. Clear and ready perception is important since the devices and systems depend in large measure upon the visual distinctiveness of a line of demarcation between reacted and unreacted portions of reagent within a tube. The color change produced in many of these devices and systems is from yellow to greenish-blue. Such a change is not always easy to perceive, especially by relatively unskilled personnel under less-than-optimum lighting conditions. There exists a clear need for a BAC system which is reliable, easy to use and relatively inexpensive.